1. Field of the Invention
The present invention generally relates to ceramic coatings deposited on components with surface holes that are required to remain open after the coating process. More particularly, this invention is directed to a method of removing ceramic coating deposits from surface holes using a laser drilling technique having parameters that reduce the incidence of delamination and cracking of the ceramic coating surrounding the surface hole.
2. Description of the Related Art
Components located in certain sections of gas turbine engines, such as the turbine, combustor and augmentor, are often thermally insulated with a ceramic layer in order to reduce their service temperatures, which allows the engine to operate more efficiently at higher temperatures. These coatings, often referred to as thermal barrier coatings (TBC), must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. Coating systems capable of satisfying these requirements typically include a metallic bond coat that adheres the thermal-insulating ceramic layer to the component, forming what may be termed a TBC system. Metal oxides, such as zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3) magnesia (MgO) or other oxides, have been widely employed as the material for the thermal-insulating ceramic layer. The ceramic layer is typically deposited by air plasma spraying (APS), low pressure plasma spraying (LPPS), or a physical vapor deposition (PVD) technique, such as electron beam physical vapor deposition (EBPVD). Bond coats are typically formed of an oxidation-resistant diffusion coating such as a diffusion aluminide or platinum aluminide, or an oxidation-resistant overlay coating such as MCrAIY (where M is iron, cobalt and/or nickel).
While TBC systems provide significant thermal protection to the underlying component substrate, internal cooling of components such as combustor liners and turbine blades (buckets) and nozzles (vanes) is often necessary, and may be employed in combination with or in lieu of a TBC. Air-cooled components of a gas turbine engine typically require that the cooling air flow is discharged through carefully configured cooling holes that distribute a cooling film over the component surface to increase the effectiveness of the cooling flow. Cooling holes are typically formed by such conventional drilling techniques as electrical-discharge machining (EDM) and laser drilling, or with complex advanced casting practices that yield castings with dimensionally correct openings. However, EDM cannot be used to form cooling holes in a component having an existing ceramic TBC since ceramic is electrically nonconducting. While laser drilling techniques are capable of forming cooling holes in a TBC-coated component, the brittle ceramic TBC surrounding the cooling hole is prone to delamination and cracking of the TBC at the interface between the TBC and the underlying metallic bond coat. Accordingly, cooling holes are often machined by EDM or laser drilling after deposition of the bond coat but prior to application of the TBC. However, the presence of TBC deposits in the cooling holes of an air-cooled component can detrimentally affect the service life of the component as a result of the TBC altering the shape and reducing the size of the cooling hole openings. Particularly for TBC's deposited by plasma spraying (APS and LPPS), a significant amount of ceramic can be deposited in the cooling holes when depositing a sufficiently thick TBC to thermally insulate hot section components. The obstruction of cooling holes with TBC not only occurs with new manufactured air-cooled components, but also when refurbishing a TBC on a component returned from the field.
From the above, it can be seen that manufacturing and refurbishing an air-cooled component protected by a TBC is complicated by the requirement that the cooling holes remain appropriately sized and shaped. Typical solutions are to limit the thickness of the TBC applied or, more preferably, perform a final operation to remove ceramic from the cooling holes in order to reestablish the desired size and shape of the openings. Various techniques have been proposed for this purpose. One approach is to employ a waterjet treatment, a notable example of which is disclosed in commonly-assigned U.S. patent application Ser. No. 10/086,266 to Farmer et al. As cooling hole diameters decrease, generally below 0.020 (about 0.5 mm) and particularly below 0.010 inch (about 0.25 mm), removal of ceramic becomes more difficult with a waterjet, especially for TBC thicknesses in excess of 0.020 (about 0.5 mm). As noted above, while conventional laser drilling techniques tend to delaminate and crack the brittle ceramic TBC surrounding a cooling hole, various laser drilling techniques have been proposed that are reported to minimize TBC damage. Commonly-assigned U.S. Pat. No. 5,216,808 to Martus et al. reports the propensity for Nd:YAG (neodymium-doped yttrium-aluminum-garnet) lasers to damage the ceramic coating surrounding a cooling hole as a result of these lasers generating beams in the infrared (IR) spectrum that thermally remove the ceramic. To avoid this problem, Martus et al. use an Excimer laser, which generates a beam in the ultraviolet spectrum, to a thermally ablate a ceramic coating from a cooling hole. Commonly-assigned U.S. Pat. No. 6,172,331 to Chen further recognizes the propensity for pulsed Nd:YAG lasers to cause TBC cracking, and as a solution utilizes a pulsed Nd:YAG laser in combination with a harmonic generator to reduce the laser beam wavelength to about 532 nanometers, which is shorter than IR wavelengths. Finally, U.S. Pat. No. 6,380,512 to Emer also discloses the use of a pulsed Nd:YAG laser to remove ceramic from a cooling hole. Emer does not alter the wavelength of the Nd:YAG laser beam used to remove the ceramic, but instead uses the laser beam at its standard IR wavelength of 1060 nm. Emer does not disclose modifying a pulsed IR laser beam to avoid the delamination and cracking of the TBC that has been reported by others in the prior art. Instead, Emer reports that the precise actual location of a cooling hole is required to effectively redrill a coated hole without damage to the component. For this purpose, Emer uses a CNC program to originally install the cooling holes and then preliminarily locate the holes for redrilling, but then requires a machine vision system to more precisely identify the actual location of the cooling holes on the component surface.